What is a
resistor ?
A resistor is an electronic
component which, as the name suggests, resists the flow of electrical
current. For a fixed applied voltage, the higher the value of resistor, the more it will resist the
flow of current. i.e. less current will flow. This relationship is
proportional and linear within normal working limits and forms the basis
of the most common calculation in electronics, namely Ohms Law. Ohms
law is shown below where V= voltage in volts, I=current in amps and
R=resistance in ohmsV = I x R
For example a current of 2 amps flowing through a resistance
of 10 ohms results in a voltage across it of 20 volts. Obviously any two
of the variables above can be considered independent so if you apply a
voltage of say 10 volts to a resistance of 5 ohms the resulting current
will be...
I = V/R = 10/5 = 2 Amps

Resistor Values
Resistors come in a very wide range of values, from less than 1
Ohm, to millions of Ohms. As the values can be so high, additional prefixes are used
For example:
1000 Ohms = 1K,
1,000,000 Ohms = 1M.
To reduce the chances of misreading
a value, the decimal point may be replaced with the letter... for example,
1.2K may be written as "1k2" , 1.2M may be written as "1M2"
and 1.2 Ohms may be written as 1R2.
Although there is an infinite possible range of resistor
values it is impractical to expect manufacturers to make and stock them.
The compromise is a set of standard values and their decade multiples.
There are two main sets of values used according to the accuracy of value
needed. These are called the E12 and E24 series. For a full description
and table of these and other series follow this link -
EIA Resistor Series

Power Rating
In the process of resisting current flow, the resistor will
create some heat. The amount of heat it will generate will increase with
current or voltage.
Every resistor has a maximum amount of heat that it can dissipate without
being damaged. This maximum is the power rating of the resistor, and is
given in Watts.
The power which the resistor will dissipate as heat can be
found from either the voltage across it or the current through it, and
it's resistance value according to the following formulae..
P = I 2 x R,
and P = V 2 /
R
where P=Power(watts), V=voltage(volts), I=current(amps) and R=resistance(ohms)

Schematic Symbols
In circuit diagrams, resistors may be shown using one of the symbols
below.